The present invention relates to a waveform equalizer for a Teletext signal and, more particularly, to an improvement in an operation timing control signal generator for use in this waveform equalizer.
Recently, in Europe, United States of America and Japan, Teletext services have been practiced, whereby textual and graphic information is inserted into television broadcast signals and this teletext signal is converted to a television signal in a TV receiver to be displayed on the screen. As shown in FIG. 1, the Teletext signal is inserted, in the form of a two-level signal and, subsequent to a color burst signal, into one or more horizontal scanning lines in the vertical blanking interval of the television signal. A data packet of the Teletext signal consists of a header and information data. According to the system in Japan, the data packet has 320 samples and the sampling clock rate is 5.73 mega samples/second.
Recently, picture ghost cancellers (waveform equalizer) have been put into practical use to remove ghost components (waveform distortion), which are caused due to multipath transmission, from a received television signal. Where the waveform distortion is caused in the television signal, due to the ghost components, it is also obviously caused in the Teletext signal.
In the case of picture ghost cancellers, the video signal has to be processed on a real-time basis to remove ghost components. For this purpose, the ghost canceller has to be designed so as to operate at high speed, which has lead them to be large in circuit scale and costly. It is economically disadvantageous to use such a picture ghost canceller to remove the ghost components from the Teletext signal having a frequency band narrower than the video signal. Further, since the above picture ghost cancellers cannot eliminate short-delay ghosts, they cannot be adapted for a Teletext signal equalizer without modification.
Since the Teletext signal is transmitted within the vertical blanking interval of the television signal, the real-time process is not always necessary to eliminate the waveform distortions. That is, the waveform equalizing process can be performed during a video signal interval (one field interval). This means that an equalizer which may be simple in arrangement and operate at low speed is enough for equalization of the waveform of the Teletext signal. An example of the waveform equalizer for the Teletext signal is disclosed in Japanese Laid-Open Patent Publication No. 56(1981)-166674.
In the Teletext waveform equalizer, various kinds of timing control signals are needed to determine the operation timings of circuits. Further, for the waveform equalizing process, about 90,000 shots of 5.73 MHz clock pulses are needed in one field interval (about 16 milliseconds). A low-speed microprocessor is not adapted for generating such high-speed timing signals. To generate such timing control signals, a conventional timing control signal generator mainly consists of TTL logic ICs including counters, flip-flops and the like. However, in this case, the number of elements constituting the counters increases, causing the circuit scale to be remarkably enlarged. Actually, about 300 general-purpose logic ICs are used.